The present invention relates to a seal structure and, more particularly, a seal structure for first and second structural members which have co-operating seal faces and are clamped together with a seal element being interposed between said co-operating seal faces, such as, for example, a seal structure for the connecting portion of the cylinder block and the oil pan, the connecting portion of the cylinder head and the cylinder head cover or the like in internal combustion engines.
In engines, the oil pan and the cylinder head cover must be mounted to the cylinder block with an oil-tight seal structure incorporated therebetween so as to avoid any leakage of oil at the connecting portion. FIG. 1 shows a typical conventional seal structure employed for the connection of the cylinder block and the oil pan, wherein 1 designates the cylinder block having a seal face 2 formed along the lower peripheral edge thereof while 3 designates the oil pan having a flange portion 4 formed along the upper peripheral portion thereof, said flange portion having a seal face 5 arranged to co-operate with the seal face 2 of the cylinder block. A seal element 6 such as an annular gasket is interposed between the two seal faces 2 and 5 and the oil pan 3 is clamped to the cylinder block 1 by a plurality of clamping bolts 8 each being passed through an opening 7 formed in the flange portion 4 of the oil pan and a corresponding opening formed in the seal element and screwed into a corresponding threaded opening 9 formed in the cylinder block 1. Conventional auxiliary elements such as a plain washer 10 and a spring washer 11 may of course be employed as shown in FIG. 1.
In order that the seal structure such as shown in FIG. 1 ensures a good seal performance for a long period of operation, a lot of conditions must be satisfied: such as that the individual clamping bolts are uniformly clamped by the same clamping torque, the initial clamping condition is stably maintained, the seal element is uniformly compressed over the whole area thereof, the compression applied to the seal element is of a desirable value which ensures the sealing performance of the seal element while it does not detract from the elasticity of the seal element, the clamping structure is immune to vibration, etc..
In usual automobile engines, the cylinder block and the oil pan are connected by 20 to 30 connecting bolts. When this number of clamping bolts are employed in a seal structure such as shown in FIG. 1, the bolt clamping process requires delicate control. For example, a sequential little-by-little tightening of the bolts does not necessarily guarantee a uniform clamping of the whole bolt system. Furthermore, the compression effected in the seal element is not necessarily proportional to the clamping torque applied to a clamping bolt. FIG. 2 shows an example of the relation between the compression effected in the seal element and the clamping torque applied to a clamping bolt. Generally, the compression for the seal element which provides the optimum sealing performance of the seal structure is in a range such as shown by A, wherein the gradient of compression/clamping torque is relatively large. This means that the clamping torque applied to the bolts must be controlled within a very narrow range, while in actual operation the possible control range for the clamping torque is approximately of an order such as shown by B. Therefore, in most cases, a compromise must be effected, so that the clamping condition of the seal structure is not always at the optimum condition over the entire region thereof. If the seal element is compressed beyond a proper compression ratio which preserves elasticity of the seal material, the seal element will soon undergo a permanent deformation, thereby losing seal performance and causing a leakage of oil. Particularly in the case of the real structure incorporated in the connecting portion of the cylinder block and the oil pan of engines, which is subject to cyclic heating and vibration, a seal element compressed in an improper stress condition is very liable to a permanent deformation. If a permanent deformation of the seal element occurs, it causes a loosening of the clamping bolts, which further accelerates the deterioration of the seal structure.